Surface acoustic-wave (SAW) sensors play an important role in many fields of material sensing and industrial applications. In general, a surface acoustic wave is an acoustic wave that propagates along the surface of a certain (piezoelectric) material. It is generated by interdigitated transducer (IDT) electrodes (or “fingers”), which are special periodic metallic bars deposited on a piezoelectric material. When any sinusoidal wave having a period equal to the period of the IDT electrodes is applied, mechanical vibration occurs beneath the IDT electrodes, thereby generating an acoustic wave, which is perpendicular to the geometry of the IDT bars. This acoustic wave propagates on the surface of the piezoelectric material away from the IDT electrodes in both directions.
The acoustic wave generated by the IDTs is localised in the surface region and penetrates the bulk piezoelectric material only to a wavelength deep region. That is why the SAW has a very high energy density at the surface, which gives the name “surface acoustic wave”. The SAW propagates in a piezoelectric material approximately 105 times slower than a regular electromagnetic wave. Consequently, the SAW wavelength in the piezoelectric material is 105 times smaller than the wavelength of an electromagnetic wave, making the SAW-based sensor a very compact device.
Fabrication of the SAW sensors requires either deposition or etching of the metallic IDTs on a piezoelectric material, and it uses the CMOS process technology, which allows a large scale manufacture. The factors that can affect the piezoelectric material surface condition include pressure, temperature, humidity and mass loading. Accordingly, SAW sensors can be used as pressure, temperature, humidity sensors, and as sensors capable of detecting mass changes or electric field alterations at the surface. A MEMS-CMOS technology facilitates the integration of the SAW sensors and their data processing circuits. Specially designed SAW sensors can also be used in a passive mode without need for batteries. An RFID antenna can be added to the input IDT electrode and the signal received by the antenna can then stimulate the SAW used for sensing as mentioned before. These are actually the SAW sensors using RFID tags. The ultrahigh sensitivity, compact nature, ease of fabrication and wireless operation make these sensors very attractive for material sensing.